This invention relates to the field of printer ribbons and more particularly to printer ribbons supplied within cartridges for ease in handling. The self-containing nature of a cartridge lends itself to holding a ribbon supply spool and takeup spool with an exposed span of ribbon. When the ribbon is exposed, there is a tendency for the ribbon to sag and become slack, thus creating kinks, folds, tangles and other undesirable problems in the cartridges. Much work has been done to provide means of tensioning the ribbon and to provide a suitable tension on the externally exposed span through the use of spring wires, shock wires and other similar devices, including supply spool drag including devices such as found in U.S. Pat. No. 4,147,439. These devices function quite adequately so long as the cartridge is attached to the drive mechanism and the takeup spool is incapable of reverse rotation.
With the advent of colored ribbons, correctable ribbons and ribbons for use on permanent type documents, such as negotiable instruments and legal documents, there is a need to switch from one type of ribbon to another during the useage of a film ribbon contained in a particular cartridge. As a result, the takeup spool is disconnected from the typewriter ribbon drive mechanism and, thus, becomes subject to reverse rotation due to ribbon tension and the resulting increased slack in the exposed ribbon span. When the slack is taken up, there is an increased risk of folds or other discontinuity on the takeup spool that will create additional problems during continued usage of the cartridge, such as retyping on ribbon previously used.
A particular problem with frictional drags on the takeup spool is that the ribbon feed mechanism must, through sheer force, overcome the full drag forces applied to the takeup spool as a brake. Drag brakes are only partially effective to prevent unspooling of the used ribbon.